The present invention relates to a ferroelectric liquid crystal display that uses a light source capable. of emitting a plurality of colors, in combination with a liquid crystal display panel, a liquid crystal optical shutter array, or a similar component, comprising a matrix of pixels having a liquid crystal layer formed from ferroelectric liquid crystals. The invention also relates to a method of driving such a ferroelectric liquid crystal display.
In the prior art, various methods have been proposed for accomplishing color display utilizing a successive additive color mixing phenomenon by using a liquid crystal cell as a shutter and by placing a light emitting device (such as an LED or CRT) behind the shutter. Prior art literature relating to such methods includes, for example, 7-9 xe2x80x9c4 A Full-Color Field-Sequential Color Displayxe2x80x9d presented by Philip Bos, Thomas Buzak, Rolf Vatne et al. at Eurodisplay ""84 (1984/9/18-20). Unlike methods that use color filters with the respective color segments provided at each pixel position, this display method produces color display by projecting differently colored lights in rapid succession. For the liquid crystal cell used with this method, the same structure as that of a cell used for monochrome display can be used. The light emitting device disposed behind the liquid crystal cell emits light of three primary colors, for example, R (red), G (green), and B (blue), which are successively projected onto the liquid crystal cell, each color for a predetermined duration of time (TS). That is, light of each color is projected onto the liquid crystal cell for the duration of time TS, in the order of R (red), G (green), and B (blue). These three primary colored lights are successively and repeatedly projected. The liquid crystal cell is controlled in synchronism with the time TS to vary the light transmittance of each display pixel. More specifically, the light transmittance for each of R, G, and B is determined by driving the liquid crystal cell in accordance with display color information. As an example, the light transmittance of the liquid crystal cell is set and held at 50% when R is being emitted for time TS, at 70% when G is being emitted for time TS, and at 90% when B is being emitted for time TS. Since the time TS is usually very short, the human eye does not perceive the respective colors as individually separate colors but as one color produced by mixing the respective colors.
Techniques utilizing such a method for ferroelectric liquid crystal display devices are disclosed in Japanese Patent Unexamined Publication Nos. 63-85523, 63-85524, and 63-85525.
In driving ferroelectric liquid crystals for color display utilizing the successive additive color mixing phenomenon, the time during which the light emitting device mounted as a light source behind the liquid crystal shutter emits light of one particular color is defined as TS, as described above. In order that changes in the color of light emitted from the light emitting device will not be perceived as flicker by the human eye when the R, G, and B colored lights are sequentially emitted from the light emitting device, the time TS must be made shorter than about 20 ms.
According to the conventional art ferroelectric liquid crystal driving method, the amount of light transmitted through a pixel during the time TS varies depending on which scan line the pixel is located. Consider, for example, a case where the entire liquid crystal display screen is displayed in white. In this case, since the color to be displayed is white, the liquid crystal is driven so that the light transmittance for each of R, G, and B becomes 100% for all pixels. During the time TS that R is being emitted, for example, a drive voltage is applied to the respective scanning electrodes. G is emitted for the next duration of time TS, followed by the emission of B for the duration of time TS, and the liquid crystal is driven accordingly for the respective durations of time TS to produce the desired color (in this case, white) for display. However, since the timing at which the selection voltage described later is applied to the selected scanning electrode becomes slightly displaced from one scanning elelectrode to the next, the length of time that the pixels on the scanning electrodes,X1, X2, . . . , Xn transmit the light of R during the time TS that the light of R is being emitted, becomes gradually shorter as the scanning progresses from top to bottom and, at the bottom most scanning electrode, pixels transmit the light of R only for a short period of time. If the length of time that a pixel transmits light, that is the amount of transmitted light, differs depending on the position of the scanning electrode associated with that pixel, the entire screen cannot be displayed with uniform brightness, nor can the color be controlled, rendering it impossible to display the desired color. For example, since the pixels on the bottommost scanning electrode transmit the light of R only for a short period of time, the amount of light of R decreases and a color different from white is displayed.
The present invention is aimed at resolving the above-described problem, and provides a ferroelectric liquid crystal display and a method of driving the same using the successive additive color mixing phenomenon for color display which can display the entire screen with uniform brightness and can achieve the display of the desired color.
According to the present invention, there is provided a ferroelectric liquid crystal display comprising: a ferroelectric liquid crystal display element which includes a ferroelectric liquid crystal that is sandwiched between a pair of substrates having a plurality of scanning electrodes and signal electrodes deposited respectively on the opposing surfaces thereof; and a light source which successively emits a plurality of different colors of light, wherein a scanning period (TS) during which the light source emits light of one of (the plurality of colors is divided into two periods, of which the first period (SC1) includes a selection period, for determining a display state and a non-selection period for holding therethrough the display state selected during the selection period, and the second period (SC2), constituting the remainder of the scanning period, includes a selection period for forcing the display state into a black display state and a non-selection period for holding therethrough the black display state selected during the selection period.
According to the present invention, there is also provided a ferroelectric liquid crystal display comprising: a ferroelectric liquid crystal display element which includes a ferroelectric liquid crystal that is sandwiched between a pair of substrates having N scanning electrodes and M signal electrodes deposited respectively on the opposing surfaces thereof; and a light source which successively emits a plurality of different colors, wherein a period (TS) during which the light source emits light of one of the plurality of colors is made up of an even number of scanning periods, wherein, in an odd-numbered scanning period, forward scanning is performed by scanning the scanning electrodes forward, starting at the first scanning electrode and progressing toward the N-th scanning electrode and, in an even-numbered scanning period, backward scanning is performed by scanning the scanning electrodes backward, starting at the N-th scanning electrode and progressing toward the first scanning electrode. The forward scanning and the backward scanning may be interchanged.
In a preferred embodiment of the ferroelectric liquid crystal display of the present invention, in a period (TS) during which the light source emits light of one of the plurality of colors, forward scanning is performed by scanning the scanning electrodes forward, starting at the first scanning electrode and progressing toward the N-th scanning electrode, and in a period (TS) during which the light source emits light of the same color the next time, backward scanning is performed by scanning the scanning electrodes backward, starting at the N-th scanning electrode and progressing toward the first scanning electrode, wherein the forward scanning and the backward scanning are repeated alternately.
According to the ferroelectric liquid crystal display of the present invention and its driving method, a uniform display can be produced with the entire display screen free from nonuniformity in brightness. Furthermore, the desired color can be displayed since the color can be controlled accurately.